In order to overcome this drawback, for some time now so-called “protected” stoppers for test-tubes have been introduced, said stoppers being provided with a cover which covers both the stopper and the top part of the test-tube.
In practice, this protected stopper consists of two parts which are secured together:                an elastomer inner body, for example made of butyl rubber (the actual stopper), which is used to close the test-tube and able to be perforated so as to allow the introduction of the syringe needle;        a cover, outside the abovementioned body, which is made of suitably rigid plastic material, for example polyethylene.        
According to the present state of the art various types of protected stoppers are known.
By way of example reference is made to the protected stopper described in European Patent No. 129,029 which is configured so that the cover has, on the inner wall facing the inner body, an annular groove inside which a corresponding relief projecting from the abovementioned inner body is inserted.
The protected stopper described above and other similar stoppers of the known type have the drawback that the body of the cover has undercuts for securing the inner body such that the operation of injection-moulding of the stopper itself is difficult and complicated.
Moreover, the presence of an undercut in the form of grooves on the wall of the cover results in the need for a sufficiently large thickness of the said wall, which is in contrast with the need to limit the extension of the stopper beyond the external diameter of the test-tube in order to keep the dimensions as small as possible and allow insertion of the assembled test-tube, i.e. complete with stopper, inside the holders or other adapters of analysis machines.
European Patent EP 0,419,490 attempted to overcome these drawbacks by securing together the components of the protected stopper using two extensions which project from the internal cylindrical surface of the cover in the direction of the longitudinal axis and which define a groove inside which an annular relief projecting from the inner body is inserted in a retaining relationship. In accordance with this solution, the bottom extension of the cover is positioned between the front part of the test-tube and the annular relief of the inner body.
This configuration involves, however, involves part of the inner body of the stopper having to extend necessarily outside the test-tube in order to form the connection with the cover. Since the length of the inner body of the stopper normally has a predefined value which cannot be modified, in that it is related to the length of the syringe needle, the frictional force between stopper and test-tube which ensures a sealed closure is consequently also reduced in a disadvantageous manner.